1. Field of the Invention
The present invention relates to a droplet discharging head and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus such as an inkjet recording apparatus has various advantages such as high image quality, flexibility in coping with high speed printing, easy selection of ink types, usability of inexpensive plain sheets, and the like. Among the image forming apparatuses, a so-called “ink-on-demand type” droplet discharging device has been in wide use for the capability of discharging ink droplets only when the ink is needed because collection of ink droplets unnecessary for recording is not required.
As a droplet discharging head used in the droplet discharging device, there is a known configuration including a nozzle opening for discharging a droplet such as an ink droplet, a liquid pressure chamber (also, referred to as a discharging chamber, a pressurization chamber, an ink flow path, or the like) which communicates with the nozzle opening, and a pressure generating unit for generating a pressure for pressurizing the ink in the liquid pressure chamber. In the droplet discharging head having such a configuration, the ink droplet is discharged from the nozzle opening by pressurizing the ink in the liquid pressure chamber using the pressure generated by the pressure generating unit.
There is a known type of a droplet discharging head in which a piezoelectric element is used as the droplet discharging head and a vibration plate that forms a wall surface of the liquid pressure chamber is deformed and displaced for discharging the droplet. As the droplet discharging head in which the piezoelectric element is used, there are known types such as a longitudinal vibration type which uses the displacement of the piezoelectric element in the d33 direction (displacement in a direction perpendicular to the electrode surface (thickness direction)), a transverse vibration type (a so-called bending mode type) which uses the displacement of the piezoelectric element in the d31 direction (displacement in a direction parallel to the electrode surface), and a shear mode type which uses shearing deformation of the piezoelectric element.
Among these types, in recent years, with the establishment of a pattern processing technology and a reduced cost due to the advancement of a semiconductor process or a technology in micromachining, an actuator configuration has been proposed to directly form a liquid pressure chamber or a piezoelectric element on a Si substrate. By using this technology, the piezoelectric element can be provided using a precise and simple method, such as lithography, so as to reduce the thickness of the piezoelectric element, thereby realizing high-speed driving.
In this configuration, the droplet discharging head includes a driving element that controls the driving of the piezoelectric element. The driving element is implemented on the substrate on which the liquid pressure chamber and the piezoelectric element are provided. In addition, the driving element is connected to each piezoelectric element by wire bonding or flip-chip bonding (refer to Japanese Patent Application Laid-open No. 2004-001366 and Japanese Patent Application Laid-open No. 2006-116767).
In a case where the driving element is implemented on the substrate using the wire bonding method, there is a lot of flexibility in configuring wiring compared to the flip-chip bonding method. However, in miniaturizing the droplet discharging head, a simple use of the wire bonding method is insufficient and piezoelectric elements have to be highly densely arranged. However, as the droplet discharging head is miniaturized, the piezoelectric elements need to be integrated at a high density. This causes problems in that wires may make contact with each other to cause shorting out of a circuit and reduction in production efficiency.
That is, a high-density arrangement of the piezoelectric elements contributes to miniaturization of the droplet discharging head, increase in the number of chips available from a wafer for use in forming the droplet discharging heads, and reduction in production cost. However, because of the above problems, the pitch between wires bonded by the wire bonding method cannot be decreased below about 60 μm, so that there is a limit in the miniaturization of the droplet discharging head.
Regarding the wire bonding method, wire bonding has to be performed for each piezoelectric element, one by one. This hinders improvement of production efficiency.
Meanwhile, when the driving element is provided on the substrate with a flip-chip implementation, the driving element is bonded to each of the piezoelectric elements through protruding electrodes (bumps) formed on the driving element. In a case where the driving element is provided on the substrate in a flip-chip implementation, the driving element and each of the piezoelectric elements can be directly bonded to each other using the bump without using the wire. Therefore, the flip-chip method is advantageous in terms of high production efficiency because it is not required to perform wire bonding for the piezoelectric elements one by one. In addition, because the flip-chip method does not use the wire bonding, shorting out of a circuit due to a high density arrangement of the piezoelectric elements can be suppressed.
Here, irrespective of whether the wire bonding method or the flip-chip method is adopted for implementing the driving element on the substrate, it is necessary to provide the piezoelectric elements at a high density so as to miniaturize the droplet discharging head. However, as the piezoelectric elements are arranged at a higher density, a frequency that the ink droplets are simultaneously discharged by simultaneous driving of a plurality of piezoelectric elements increases. Due to the simultaneous operation of the piezoelectric elements, a voltage drop may occur, causing a variation in the amounts of ink droplets to be discharged.
In addition, the voltage of the driving signal for the droplet discharge is likely to be lowered for the piezoelectric element that is provided away from the connection terminal to which the driving signal is externally input. Therefore, if a plurality of piezoelectric elements arranged in a predetermined direction is simultaneously driven, the voltage applied to drive the piezoelectric element located farther from the connection terminal is likely to be lowered and hence a voltage drop is more likely to occur.
From the viewpoint of implementing a thin device, an electrode of the piezoelectric element formed through thin-film forming techniques such as sputtering, vacuum deposition, chemical vapor deposition (CVD), and the like has a small thickness and thus has a relatively high resistance value. Accordingly, the problems described above are likely to occur in this type of electrodes.
As a method for solving such a voltage drop problem, Japanese Patent Application Laid-open No. 2004-001366 discloses a technology which connects a common lead electrode to a common electrode of the piezoelectric elements. This common lead electrode is a wiring electrode, extending from a portion of the pressure generating chamber except for an end portion in the parallel arrangement direction to an area outside of the pressure generating chamber. In addition, in Japanese Patent Application Laid-open No. 2004-001366, each of the common lead electrodes is connected using a connection wire formed using a wire bonding method. As a result, a resistance reduction portion including the common lead electrode and the connection wire is provided so that a resistance value of the common electrode is substantially reduced when a voltage is applied to the piezoelectric element.
However, in the configuration of Japanese Patent Application Laid-open No. 2004-001366, it is necessary to form the common lead electrode and the bonding wire, and to additionally provide a connection portion on the substrate for connecting the electrode and the wire thereto. Therefore, the area of the droplet discharging head increases, and thus it is difficult to facilitate miniaturization of the droplet discharging head. In addition, there has been a problem in that a production process is made complicated by the connection using the wire bonding.
Therefore, in the related art, it has been difficult to miniaturize the droplet discharging head, to improve the production efficiency, and to suppress the occurrence of the voltage drop.
Thus, there is a need to provide a droplet discharging head and an image forming apparatus capable of allowing the miniaturization of a droplet discharging head, improving the production efficiency, and suppressing the occurrence of the voltage drop.